Devices used to replace various joints of the human body are often implanted without the use of bone cement. To achieve and maintain long-term fixation and stability, these implants generally require some degree of bony on-growth or in-growth. The bony on-growth or in-growth necessary to promote and encourage the growth of surrounding bony and soft tissues, as well as to achieve desirable long-term fixation and stability properties, is often enhanced by fabricating porous coatings into one or more surfaces of the implant. Depending on the various features of the fabricated porous coatings (e.g., their pore size and roughness characteristics), the resulting osteoconductive properties of the implant can be improved in such a manner that the porous surfaces are able to function as scaffolds exhibiting desirable load-bearing strengths at the implantation site.
While several orthopedic device companies commercially offer implants having porous surfaces, these products largely fail to adequately replicate the trabecular structure of bone. Additionally, when an implant is designed for a specific anatomic site, its interaction with the bone is limited to the areas immediately surrounding the implantation site. Bone architecture consists of trabeculae that are oriented in certain patterns in order to optimize the bone performance in that anatomic location. Since the magnitude and mode of differential loading to which a bone is subjected is influenced by the bone's anatomic location, by Wolff's law, trabecular struts in bone can also be expected to have anatomically site-specific architectures.
Over the past few years, additive manufacturing and free-form fabrication processes have experienced some significant advances in terms of fabricating articles directly from computer controlled databases. For instance, rapid prototyping techniques allow many articles (e.g., prototype parts and mold dies) to be fabricated more quickly and cost effectively than conventional machining processes that require blocks of material to be specifically machined in accordance with engineering drawings.
Illustrative modern rapid prototyping technologies include laser based additive manufacturing processes such as selective or direct metal laser sintering processes. These processes utilize digital electronic file formats (e.g., STL files) that can be printed into three-dimensional (3D) CAD models, and then utilized by a prototyping machine's software to construct various articles based on the geometric orientation of the 3D model. The constructed articles are produced additively in a layer-wise fashion by dispensing a laser-fusible powder one layer at a time. The powder is fused, re-melted or sintered, by the application of laser energy that is directed in raster-scan fashion to portions of the powder layer corresponding to a cross section of the article. After each layer of the powder is fused, an additional layer of powder is dispensed, and the process repeated, with fused portions or lateral layers fusing so as to fuse portions of previous laid layers until the article is complete.
Additive manufacturing processes allow for highly complex geometries to be created directly (without tooling) from 3D CAD data, thereby permitting the creation of articles exhibiting high resolution surfaces. While these processes have been useful for detailing various surface properties of produced articles, such processes have struggled to replicate surfaces having reduced three-dimensional structural densities. For instance, such processes are unable to adequately replicate articles having randomized porous or partially randomized porous metallic structures, including metal porous structures having interconnected porosity. As such, there is a need for an additive manufacturing process that can replicate articles having reduced three-dimensional structural densities, including porous and partially porous metallic structures.
The present invention is intended to improve upon and resolve some of these known deficiencies of the art.